As modern commerce depends on reliable and cost-effective methods for delivering products from suppliers to users, the availability of durable and reliable highways, roads and other support surfaces for vehicles is vital for sustaining a modern economy. To provide better support surfaces, highways, roads, and sidewalks are commonly paved with an asphalt concrete that is laid over the surface of the sub-base. Asphalt concrete cement is preferred over Portland Cement Concrete to pave roads because it is less expensive and very durable. Asphalt hot mix can also be constructed at night, which allows major roads to be shut down at the least busy of times for maintenance. Relative to road noise, asphalt is also quieter than Portland Cement Concrete, making it the better choice for roads.
Asphalts are essentially mixtures of bitumen, as binder, with aggregate, in particular coarse and fine aggregate and mineral fillers. There are many different types of asphalts available and their characteristics can vary quite significantly. The design of asphalts for bituminous paving application is a complex process of selecting and proportioning materials to obtain the desired properties in the finished construction while minimize undesirable characteristics.
In evaluating the development of proper mix designs, the aggregate gradation, binder content, and compaction effect are critical in obtaining a hot mix asphalt that performs according to the requirements. The final goal of mix design is to achieve a balance among all of the desired properties. Both unmodified and polymer modified asphalt binders have been investigated for to help achieve the desired performance properties in the hot mix asphalt, such as resistant to permanent deformation, resistance to low temperature thermal cracking and resistance to fatigue cracking.
Unsaturated thermoplastic elastomers like styrene-butadiene-styrene (SBS) block copolymers are polymers used for asphalt modification. They enhance the elastic recovery capacities of asphalt and, therefore, its resistance to permanent deformations. However, unsaturated elastomeric polymers are quite expensive and are subjected to degradation when exposed to atmospheric agents and mechanical stress. This can result in a significant cost increase for the product. While SBS is recognized for performance benefits, research has focused on most cost effective modifiers in exchange for sacrificing superior performance.
Olefinic polymers have been investigated for use as modifiers. They are available in large quantities with different rheological properties and at low cost. Polyethylene (PE) and polypropylene (PP) are plastomers. They bring a high rigidity (i.e., lack of elasticity, resistance to bending) to the product and significantly reduce deformations under traffic load. Due to their chemical and structural make-up, PE and PP suffer from the drawback that they are almost completely immiscible with asphalt, and are thus limited in use.
Unmodified asphalts will flow at high temperature and are limited with respect to certain road applications. The characteristics of road asphalts can be improved by incorporating of elastomeric-type polymers. There exists a wide variety of polymers that can be mixed with asphalt. Of these, SBS is a commonly used polymer in asphalt modification. The modified asphalts thus obtained commonly are referred to variously as bitumen/polymer binders or asphalt/polymer mixes. There is a need for a modification to hot mix asphalt concrete mixes that would increase the resistance to permanent deformation at high temperatures while improving the modulus of the mix at intermediate temperatures without affecting the binder properties significantly to improve the fatigue properties.
The bituminous binders, even of the bitumen/polymer type, which are employed at the present time in road applications often, do not have the optimum characteristics at low enough polymer concentrations to consistently meet the increasing structural and workability requirements imposed on roadway structures and their construction. In order to achieve a given level of modified asphalt performance, various polymers are added at some prescribed concentration. It is common practice to use a cross-linking agent, such as sulfur, to compatibilize polymer in the asphalt and help improve the properties. The sulfur in these cases is added in a range of about 3-5 wt. % based on the polymer. When sulfur is added at a loading greater than several weight percentage, then it is considered a modifier.
When added to bitumen at 140° C., sulfur is finely dispersed in bitumen as uniformly small particles; coagulation and settlement of sulfur particles become noticeable after a few hours. Therefore, the sulfur extended asphalt (SEA) mixtures can be produced directly in the mixing plant just before the laying of the asphalt mixture. One major concern in handling sulfur-asphalt mix is the fear of the evolution of hydrogen sulfide (H2S) during production and laying. This problem can be ameliorated by adding carbon or ash to sulfur. H2S evolution starts at temperatures higher than 150° C., so that the application at temperatures up to 150° C. avoids pollution and safety problems. However, H2S evolution starts well below 150° C., i.e. about 130° C. Also, higher loadings of sulfur cannot be used in conjunction with unsaturated polymers, such as SBS, SB and SBR because of the formation of gel and gross incompatibility.
Besides concerns related to emissions, sulfur modified asphalt compositions have not able to be super pave performance graded (PG). The unreacted sulfur in the past tended to cause embrittlement upon aging. Earlier reported sulfur asphalt binders did not report on improving the grading of the asphalt over an unmodified control at both the high temperature and low temperature regions.
A need exists for asphalt compositions that can be performance grade. It would be advantageous if the compositions could include materials, such as sulfur, that enhance performance of the compositions, while being relatively inexpensive.